Free-space optical transmitters are typically required to provide a high-power, single-polarization output. An optical amplifier capable of providing a high-power, single-polarization output is presented in a paper, Hakimi et al., xe2x80x9cHigh-power Single-polarization EDFA with Wavelength Multiplexed pumpsxe2x80x9d, CLEO ""98, CWK1, 1998.
High-power optical amplifiers, such as erbium-doped fiber amplifiers (EDFA""s), are often employed in free-space communications. Single-polarization EDFA""s provide additional capabilities for polarization diplexing and improved communication performance, since orthogonally polarized amplified spontaneous emission (ASE) can be eliminated at both receiver and transmitter. Such features can offer substantial benefit in free-space links, where an improvement in single-polarization receiver sensitivity directly reduces required transmitter power.
High gain enables a power amplifier to output its maximum saturated output power over an extended range of input power levels. In master oscillator power amplifier (MOPA) designs, this feature makes the transmitter less sensitive to insertion loss changes in the elements leading up to the power amplifier. Furthermore, saturated EDFA""s are average power limited (APL), and, therefore, peak output power is inversely proportional to the duty-cycle of the input.
FIG. 1 is a schematic diagram of the erbium-doped fiber amplifier (EDFA) 100a by Hakimi et al., which is a two-stage, double-pass, polarization-maintaining EDFA with a high-power, saturated output composed of reliable, commercially available components. FIGS. 2-5 further illustrate the EDFA of FIG. 1.
Referring now to FIG. 1, a 1550 nm input optical signal 105 is coupled to a standard, single-mode, optical fiber 110 via a polarization beam splitter (PBS) 107. The input signal 105 travels in a forward pass through the optical fiber 110, among other optical media. The EDFA 100a of FIG. 1 achieves a high output power level by multiplexing four pump lasers 125 together with polarization-insensitive, dense, WDM, fused tapered couplers 120. See M. N. McLandrich et al., J. Lightwave Technol. 9, 442-447 (1991). The pump diode laser wavelengths range from 970 nm to 985 nm, with 5 nm of separation between each. According to Hakimi et al., at the recommended operating current, the nominal power of each pump before the WDM combiner is 90 mW, and the output pump power from each set of four combined pump lasers is 330 mW. The output from the dense WDM 120 is coupled to the optical medium, carrying the input signal 105, by a standard WDM 115.
In the forward pass, the output from the WDM 115 (i.e., the input signal 105 and signals from the diode pumps 125) encounters an erbium-doped fiber 130. The 970 nm-985 nm outputs from the four diode pumps 125 get absorbed by the erbium-doped fiber 130, increasing the energy level in the erbium-doped fiber 130. Unlike the 970 nm-985 nm outputs from the diode pumps 125, the 1550 nm input signal 105 passes through the erbium-doped fiber 130, increasing in power as a result of encountering the erbium-doped fiber 130 charged by the four diode pumps 125. Because the input signal 105 is amplified by the erbium-doped fiber 130, the erbium-doped fiber 130 is often referred to as an optical gain medium.
FIG. 2 is a schematic diagram representing how the erbium-doped fiber 130, in combination with the output energy from the four diode pumps 125, provides gain to amplify the input signal 105. As stated above, the pump energy 205 is absorbed by the erbium-doped fiber 130. The energy absorption representation 210 indicates that the energy in the erbium-doped fiber 130 reaches a peak level, Emax. The energy relaxes from its maximum potential energy level to a nominal potential energy level, Enom, as indicated by an energy relaxation representation 215. In this manner, pump energy is used to transfer energy from the ground state, Emin, to the excited state Enom, creating an energy inversion. Enom corresponds to the energy of the input signal wavelength. Then, when the input signal 105 encounters the excited-state erbium-doped fiber 130, stimulated emission occurs, as indicated by a stimulated emission representation 220.
As a result of the release of optical energy resulting from the stimulated emission process, a signal photon 225 of 1.5 xcexcm wavelength entering the energy-enhanced erbium-doped fiber 130 exits as multiple photons 230 of the same wavelength, thus amplifying the signal.
Referring again to FIG. 1, a second WDM 115 and erbium-doped fiber 130 are encountered by the input signal having been once amplified. Following the second erbium-doped fiber 130, the twice amplified input signal encounters a Faraday mirror 135, causing the twice amplified input signal to travel in a reverse pass with a 90 degree polarization rotation. The signal traveling in the reverse pass is amplified a third and a fourth time, as it traverses through the optical gain mediums 115 to the polarization beam splitter 107. The output 140a is the saturated output from the EDFA 100a. 
The input port of the EDFA 100a, consisting of the fiber-pigtailed polarization beam splitter 110, has a 0.4-dB port-to-port loss. The Er-fiber (i.e., erbium-doped fiber) splice losses, Faraday mirror loss, and 980/1550 WDM losses are 0.1 dB, 0.33 dB, and 0.1 dB, respectively. Each amplifier stage uses approximately 15 meters of conventional erbium-doped fiber.
FIG. 3 shows the output power verses the input signal power for the EDFA 100a measured after the polarization beam splitter 107. The curve defined by the triangles is a result of the energy pumps providing 330 mW/stage (triangles). In this case, the output power of the EDFA 100a is just over 255 mW at 1556 nm, which is near the peak of the gain. When the pump lasers are turned up above the recommended operating point to 400 mW/stage (diamonds), the saturated output power is 315 mW.
FIG. 4 illustrates the wavelength dependence of the output of the EDFA 100a. A 1 mW input signal and 330 mW/stage pump power are held constant in generating the response curve. The output power is above 240 mW over a 30 nm range.
FIG. 5A is a transfer function 500 of a typical amplifier, of which the EDFA 100a is a member. The transfer function has two regions: a small signal gain region 505 and a saturated output gain region 510. The gain curve 515 in the small signal gain region 505 increases at a typical rate, where Pout equals go*Pin, and go is the small signal gain. Note that, in dB, the linear expression Pout=go*Pin transforms to Pout [dB]=(go+Pin) [dB]. In the saturated output gain region 510, the curve 515 asymptotically increases to Psat_out. The EDFA 100a of FIG. 1 operates entirely in the saturated output gain region 510 for input power levels about xe2x88x9215 dBm. However, for input power levels of below xe2x88x9215 dBm, the EDFA 100a becomes unstable, oscillating instead of outputting a constant power level for an input signal of constant power level. The EDFA 100a does not simply revert to a small signal gain amplifier, as might be suggested by the transfer function, because the EDFA 100a is designed only to operate stably for input power levels that drive the amplifier far into the saturated output gain region.
FIG. 5B is a transfer function of gain versus output power corresponding to the transfer function of FIG. 5A.
A fault-tolerant, loss-insensitive region in a two-stage, double-pass, polarization maintaining (PM) EDFA has been identified, in which assertion of optical elements can be used for, among other reasons, to improve stability, output power, and efficiency of the EDFA. Employing the principles of the present invention, stable, greater than 0.5 Watt output power levels are obtainable for input power levels to the EDFA spanning over a 30 dB dynamic range, and 980/1550 nm conversion efficiencies that can exceed 42%.
In one embodiment of the present invention, an optical amplifier includes an amplifying optical path of elements of low insertion losses, including a non-lasing optical gain medium through which an optical signal is amplified. The optical amplifier further includes an optical return path by which amplified light from a first pass returns through the gain medium in a reverse pass through the gain medium. At least one optical element optically disposed in the optical return path, between the forward pass and the reverse pass, has a loss substantially greater than the insertion losses of the non-gain elements within the amplifying optical path. The amplifying optical path is sometimes referred to as the loss-sensitive region, and the region beyond the amplifying optical path (i.e., between the forward pass and reverse pass) is sometimes referred to as the loss-insensitive region (see FIG. 6, 600). Non-gain optical elements may include, for example, optical coupling elements (e.g., polarization beam splitters, circulators, or wavelength multiplexers) and splices.
The elements in the amplifying optical path are typically selected to have insertion losses less than about 0.5 dB, preferably less than about 0.2 dB. The optical element(s) between the forward pass and the reverse pass can have insertion loss(es) of greater than 0.5 dB without significant effect on the output power of the optical amplifier.
The optical return element, used to direct the input signal into the optical amplifier and direct the amplified signal out of the optical amplifier, is typically a beam splitter, polarizing beam splitter, or circulator.
In the preferred embodiment, each gain medium includes at least one erbium-doped fiber. To increase the available energy in the gain medium, the amplifier includes at least one energy pump coupled to the gain medium. For an erbium-doped fiber, the energy pump is typically chosen to output a 980 nm wavelength signal. For the 980 nm pumps wavelengths composed of about 500 mW of wavelength multiplex pump power, pumping a Lucent(copyright) HP980 erbium fiber, the erbium-doped fiber is preferably between about 15 and 17 meters in length. The length of the fiber is dependent on type of fiber and pump power levels.
The optical element(s) in the loss-insensitive region include(s) at least one of the following elements: a band pass filter, band reject filter, notch filter, comb filter, beam shaper, at least one wavelength division multiplexer coupling at least one 1480 nm wavelength output signal to a respective gain medium or at least one 980 nm WDM or pump element coupling that can be used for reverse pumping or redirecting excess forward pump energy, such as a cladding pump. Further, the optical return element is selected from a group consisting essentially of: a mirror, Faraday mirror, polarization rotation reflection element, beam splitter and plural mirrors, or continuous optical medium loop. In contrast to a laser, in the EDFA, the round-trip gain of the optical signal is less than the round-trip loss of the optical signal. In other words, the EDFA amplifies the input signal, and it is desirable to minimize frequencies at which lasing might occur.
In one implementation, the amplifying optical path of elements includes an input element, first WDM and second WDM in the loss-sensitive region, third WDM in the loss-insensitive region, at least one optical filter also in the loss-insensitive region, and optical return element. In this implementation, the input element is a polarizing beam splitter that directs the input signal into the optical medium of the amplifier and out of the optical medium of the amplifier after a double-pass through two optical gain mediums. The first WDM inserts into the first non-lasing optical gain medium a 980 nm signal from at least one energy pump. The second WDM inserts a 980 nm signal from at least one energy pump into the second non-lasing optical gain medium. The third WDM inserts into the first and second optical gain media a 1480 nm signal from at least one energy pump. The optical filter in the loss-insensitive region (i) has an insertion loss that can be substantially greater than the insertion losses of optical elements external from the loss-insensitive region and (ii) passes essentially the optical signal. In this implementation, the optical return element is preferably a Faraday mirror. The optical signal that has been amplified by the optical amplifier of this embodiment is substantially singularly polarized through use of the polarizing beam splitter and Faraday mirror. This implementation allows for multiple spare energy pumps in both the loss-sensitive and loss-insensitive regions and at both 980 nm and 1480 nm wavelengths.
In one embodiment, at least one of the optical elements in the optical return path between the forward pass and the reverse pass restricts the optical signal to wavelengths of interest to prevent other wavelengths from lasing, thereby increasing stability. The optical amplifier is suitable for use in free-space and fiber optic network applications.